An Introduction to Environmental Chemistry

The Chemistry of Continental Waters 163

same redox potential as nitrate reduction), these oxides may be reduced to soluble
Fe(II) and Mn(II) (see Table 4.7). Indeed, iron is soluble only under low redox
or acidic conditions (Box 5.4).
Sulphate reduction (see Table 4.7) is not an important mechanism of organic
matter respiration in freshwaters because dissolved sulphate levels are usually low.
In seawater, however, sulphate is abundant and sulphate reduction is very impor-
tant (see Section 6.4.6). Methanogenesis (see Table 4.7) can be an important res-
piration process in some organic-rich freshwater lake and swamp sediment. The
reduced reaction product, methane (CH 4 ), a greenhouse gas (see Section 7.2.4),
is known to bubble out of some wetlands, including rice paddy fields, contribut-
ing significantly to atmospheric CH 4 budgets (see Section 3.4.2).

5.5.1 Nutrients and eutrophication

In addition to CO 2 , water and light, ions (or nutrients) are needed for plant
growth. Some of these ions, for example Mg^2 +, are abundant in freshwater, but
other essential nutrients, for example nitrogen (N) and phosphorus (P), are
usually present at low concentrations in natural systems. If light availability does
not limit algal growth, chemical limitation is likely to occur when demand for
nitrogen and phosphorus exceeds their availability. Consequently, a great deal of
attention has been focused on the behaviour of nitrogen and phosphorus in
natural waters and their role as potential, or actual, limiting nutrients. In seawater,
the ratio of nitrogen to phosphorus required for optimal growth is quite well
known, being 16 : 1 on an atomic basis. In freshwater, the required nitrogen : phos-
phorus ratio (N : P) is more variable. If, however, either nitrogen or phosphorus
are in excess of the ratio required for optimal growth, it follows that the less abun-
dant nutrient may be totally consumed and become limiting. Of course, nutrient
elements can be available in excess when artificially introduced into the environ-
ment, for example as nitrate- and phosphate-based fertilizers. Nitrogen and phos-
phorus that leach from fertilizer applications often stimulate exessive algal
growth (biomass) in water courses, a problem referred to as eutrophication. The
exess algal biomass can cause toxicity, clogging of water filters, unsightly water
bodies, reduced biodiversity and low oxygen concentrations in stratified
waters.

Phosphorus

In natural waters, dissolved inorganic phosphorus (DIP) exists predominantly as
various dissociation products of phosphoric acid (H 3 PO 4 ) (eqns 5.3–5.5). Phos-
phorus is usually retained in soils by the precipitation of insoluble calcium and
iron phosphates, by adsorption on iron hydroxides or by adsorption on to soil
particles. As a result, DIP in rivers is derived mainly from direct discharges, for
example sewage. DIP concentrations vary inversely with river flow (Fig. 5.10),
the input being diluted under higher flow conditions. Since phosphate is usually
in sediments as insoluble iron(III) phosphate (FePO 4 ), under reducing conditions
(such as occur in sediments when oxygen consumption exceeds supply (Section